Mask valve



FIG. 1 is a front perspective view of a mask valve, showing my design.

FIG. 2 is a rear perspective view thereof.

FIG. 3 is a front elevation view thereof.

FIG. 4 is a rear elevation view thereof.

FIG. 5 is a left side elevation view thereof.

FIG. 6 is a right side elevation view thereof.

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a bottom plan view thereof.

The broken lines in the drawings depict portions of the mask valve that form no part of the claimed design. 

CLAIM The ornamental design for a mask valve, as shown and described. 